The present invention relates to a thin film transistor and a static RAM using a thin film transistor.
In the recent semiconductor technology, there is known a thin film transistor of the type that it is formed on an interlayer insulating film covering elements formed on the surface of a semiconductor substrate.
FIG. 9 is a cross sectional view showing the structure of a thin film transistor as a load transistor of an SRAM. A semiconductor thin film 10 is formed on an interlayer insulating film (not shown) on a semiconductor substrate (not shown). A source region 12 and a drain region 13 are formed on opposite end portions of the semiconductor thin film 10, and a channel region 14 is formed between the source and drain regions. On the channel region 14, there is formed a gate oxide film 15 on which a gate electrode 11 is formed.
A thin film transistor shown in FIG. 10 has its gate electrode 11 and gate oxide film 15 formed under the semiconductor thin film 10, and is symmetrical to the device shown in FIG. 9 relative to the semiconductor thin film 10.
An offset region 16 having a width x is provided between each drain region 13 and gate electrode 11. This offset region 16 is provided to reduce a standby current to be consumed during an inactive state of the thin film transistor. The provision of such an offset region 16 weakens the electric field between the drain region 13 and gate electrode 11, reducing the leakage current.
The offset region 16 cannot be controlled by a voltage applied between the drain region 13 and gate region 11. As a result, the offset region 16 functions as a resistor during an active state of the thin film transistor. This causes the drive current of the thin film transistor to lower, and the variation of resistance values of offset regions may change the characteristics of thin film transistors.